Water can be used for a variety of things. It can be used for drinking and can also be used to have a bath or a shower. But when you are running a bath or getting a glass of water from the tap, do you ever wonder why we really need it? It’s not just for quenching our thirst or having a nice relaxing soak - alongside oxygen, sunlight and food, it is one of the most important substances on Earth because without it, we would not be here!

In comparison to the other planets in the Solar System, we are very lucky to have such a large percentage of water covering our Earth. Just imagine how difficult it would be to get a drink of water on the hot, dry surfaces of Mercury and Venus - you would have a pretty tough time! Since water is important to all living things, it is one of the things that astronomers search for signs of when they study other planets. Can you figure out why? That’s right; they think that if there is water on other worlds, then there might be aliens living on them! How exciting would that be? What would you say to another being who lived on another planet?

Astronomers have not just searched within the orbit of Pluto for life, they have peered even further outside of it - seemingly with not much luck for quite some time. Hunting for planets as a whole is extremely exciting; it is almost like going on an Easter Egg hunt - you never know what type of planet you might get! So imagine their surprise when scientists came across something quite unusual - they had not found a planet with water flowing on its surface, but a watery moon in our own Solar System. You might have been told which natural satellite this is by your parents or teachers. Can you remember what the moon was called and which planet it belongs to? Here’s a clue; the moon is named after the queen of Crete in Greek mythology and it belongs to a stripy planet that is so large that it is more than twice the mass of all of the planets in the Solar System combined. Without peeking, write down what you think the answer might be and carry on reading to see if you are right!

This moon, if you have not already guessed, belongs to the gas giant Jupiter and is called Europa. Europa was first discovered by the astronomer Galileo Galilei in 1610 when he looked through his telescope. However, he did not just find this watery moon, he also found three others - today we know them as Io, Ganymede and Callisto and they are known as Galilean satellites - they are probably the most famous of Jupiter’s 64 moons! Europa is the sixth closest moon to its gaseous parent and is the smallest of the Galilean satellites - it is even tinier than our Earth-orbiting Moon! Despite being a long way from our planet, we have learned more and more about Europa with the help of powerful Earth-bound telescopes along with space probes that have flown passed its surface.

Jupiter's four largest (Galilean) moons, from left to right: Io, Europa, Ganymede, Callisto.
Image: NASA

One of the things that astronomers have learned is that it always seems to be like a wintery wonderland on Europa due to its scarred, yet smooth, icy surface. While the moon has oxygen in its atmosphere, amazing views of the Solar System, along with the fun of being able to go ice-skating everyday on its surface, living on Europa would not be comfortable. For one, you would still have to wear a spacesuit and you would also need to wrap up very warm indeed - think about how many layers you wore on the coldest day that you’ve experienced on Earth and then multiply it by about six! It is hardly surprising as Europa, which lies at an average distance of around 483 million miles from the Sun, is incredibly chilly at an average temperature of -160 degrees Celsius (-256 degrees Fahrenheit) at the equator and an even colder -220 degrees Celsius (-364 degrees Fahrenheit) at its poles! Europa’s surface is frozen solid, but astronomers believe that if you were to get an ice pick and were to chip away at it, then you would find that this tiny moon is very rocky underneath. Tunneling further, you might find the iron core that scientists think exists at the heart of this wintry world. You can find an iron core at the centre of our own planet which creates a magnetic field that protects us from harmful space radiation and astronomers think that the same might be true for the moon. Being similar to our planet in some ways, astronomers can only imagine what would be able to survive on Europa. Would these aliens look like us or would they look completely different? Would they be able to survive on the surface or would they tunnel underneath it? The last question became important when scientists released that Europa could have an ocean under its frozen crust.

Europa's frozen, icy surface. Image: NASA

As you might have found out by accidentally swallowing a mouthful of sea water when you have gone to the beach, Earth’s oceans are very salty, and that is exactly what astronomers expect on Europa - an ocean that is full of salt. You might think that Earth has lots of water, but scientists think that there is a larger volume on Europa with a depth of 60 miles (97km)!

Astronomers are so amazed by Jupiter’s moon that they hope to one day visit Europa. However, a walk on this icy satellite is extremely dangerous and it’s not just due to the slippery ice that you could fall over on! The radiation belts of Jupiter would occasionally zap you with a deadly high dosage of radiation that would make you incredibly ill. If there were aliens living on Europa, then they would either have to wear an extremely protective layer of clothing made of lead or they would have to live under the surface in the ocean. If they were based in the water, then they might be like the creatures that we find in the depths of our seas.

Unfortunately, however, there has been no evidence for life on Europa as yet, but that does not mean that astronomers are going to stop looking. This moon is probably one of the most likely places in our Solar System to find life due to its oceans, and it is in the deepest regions of these waters that their hunt continues - along with the help of space probes. If you were in charge of sending a probe to Europa, what would you call it and what do you think are the most important instruments to have on board when looking for life? One of your answers might be a radar that would allow you to penetrate under the surface, or another might be to crash your probe into the moon so that you could collect samples of ice to search for alien traces. Scientists have thought of some very ambitious space probes - some with drills on them to tunnel through the surface to reach the ocean. Maybe you could have a guess as to what life might look like on Europa - but remember, this life form must either be able to live under water or it should be able to survive very cold temperatures along with the intense radiation from Jupiter. Or who knows, maybe your alien is prepared to survive both!

Europa orbits its parent planet, Jupiter, once every 3.5 Earth days. Europa is also phase locked so that the same side is always facing Jupiter, just like the same side of the Moon is always facing Earth.